What's the difference between "grep -e" and "grep -E" [closed]

Question

I have a file test.txt, in which there are some formatted phone numbers. I'm trying to use grep to find the lines containing a phone number.

It seems that grep -e "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txt doesn't work and gives no results. But grep -E "[0-9]{3}-[0-9]{3}-[0-9]{4}" test.txtworks. So I wonder what's the difference between these 2 options.

According to man grep:

-E, --extended-regexp Interpret pattern as an extended regular expression (i.e. force grep to behave as egrep).

-e pattern, --regexp=pattern Specify a pattern used during the search of the input: an input line is selected if it matches any of the specified patterns. This option is most useful when multiple -e options are used to specify multiple patterns, or when a pattern begins with a dash (`-').

But I don't quite understand it. What is an extended regex?

Answer 1

As you mentioned, grep -E is for extended regular expressions whereas -e is for basic regular expressions. From the man page:

EDIT: As Jonathan pointed out below, grep -e "specifies that the following argument is (one of) the regular expression(s) to be matched."

Basic vs Extended Regular Expressions

In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification. For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax error in the regular expression. POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

But man pages are pretty terse, so for further info, check out this link:

http://www.regular-expressions.info/posix.html

The part of the manpage regarding the { meta character though specifically talks about what you are seeing with respect to the difference.

grep -e "[0-9]{3}-[0-9]{3}-[0-9]{4}" 

won't work because it is not treating the { character as you expect. Whereas

grep -E "[0-9]{3}-[0-9]{3}-[0-9]{4}" 

does because that is the extended grep version — or the egrep version for example.